Aeronautical Information Manual (AIM), page 469

Index   468 -- Page 469 -- 470


(1) Automated "REMARKS."

[a] Density Altitude.

[b] Variable Visibility.

[c] Variable Wind Direction.
(2) Manual Input "REMARKS."

[a] Sky Condition.
[b] Visibility.

[c] Weather and Obstructions to Vision.
[d] Temperature.

[e] Dew Point.
[f] Wind; and

[g] Altimeter Setting.

"Remarks ... density altitude, two thousand five hundred ...
visibility variable between one and two ... wind direction
variable between two four zero and three one zero ...
observer ceiling estimated two thousand broken ...
observer temperature two, dew point minus five."
d. Automated Surface Observing System
(ASOS)/Automated Weather Sensor System
(AWSS). The ASOS/AWSS is the primary surface
weather observing system of the U.S. (See Key to
Decode an ASOS/AWSS (METAR) Observation,
FIG 7-1-5 and FIG 7-1-6.) The program to install
and operate these systems throughout the U.S. is a
joint effort of the NWS, the FAA and the Department
of Defense. AWSS is a follow-on program that
provides identical data as ASOS. ASOS/AWSS is
designed to support aviation operations and weather
forecast activities. The ASOS/AWSS will provide
continuous minute-by-minute observations and
perform the basic observing functions necessary to
generate an aviation routine weather report (ME-

TAR) and other aviation weather information. The
information may be transmitted over a discrete VHF
radio frequency or the voice portion of a local
NAVAID. ASOS/AWSS transmissions on a discrete
VHF radio frequency are engineered to be receivable
to a maximum of 25 NM from the ASOS/AWSS site
and a maximum altitude of 10,000 feet AGL. At many
locations, ASOS/AWSS signals may be received on
the surface of the airport, but local conditions may
limit the maximum reception distance and/or altitude.
While the automated system and the human may
differ in their methods of data collection and

interpretation, both produce an observation quite
similar in form and content. For the "objective"

elements such as pressure, ambient temperature, dew
point temperature, wind, and precipitation accumula-
tion, both the automated system and the observer use
a fixed location and time-averaging technique. The
quantitative differences between the observer and the
automated observation of these elements are
negligible. For the "subjective" elements, however,
observers use a fixed time, spatial averaging
technique to describe the visual elements (sky
condition, visibility and present weather), while the
automated systems use a fixed location, time
averaging technique. Although this is a fundamental
change, the manual and automated techniques yield
remarkably similar results within the limits of their
respective capabilities.

1. System Description.

(a) The ASOS/AWSS at each airport location
consists of four main components:
(1) Individual weather sensors.

(2) Data collection and processing units.
(3) Peripherals and displays.
(b) The ASOS/AWSS sensors perform the
basic function of data acquisition. They continuously
sample and measure the ambient environment, derive
raw sensor data and make them available to the
collection and processing units.

2. Every ASOS/AWSS will contain the
following basic set of sensors:
(a) Cloud height indicator (one or possibly

(b) Visibility sensor (one or possibly three).
(c) Precipitation identification sensor.

(d) Freezing rain sensor (at select sites).

(e) Pressure sensors (two sensors at small
airports; three sensors at large airports).
(f) Ambient temperature/Dew point tempera-
ture sensor.

(g) Anemometer (wind direction and speed
(h) Rainfall accumulation sensor.

3. The ASOS/AWSS data outlets include:
(a) Those necessary for on-site airport users.

Meteorology 7-1-25

Page 469 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   468 -- Page 469 -- 470